Coating for the protection of ferrous base alloys at elevated temperatures



United States Patent 3,301,702 COATING. FOR THE PROTECTION OF FERROUS BASE ALLOYS AT ELEVATED TEMPERATURES Stuart L. Ames, Natrona Heights, Frank A. Malagari, Jr., Freeport, and Jesse R. Conner, Jr., Sarver, Pa., assignors to Allegheny Ludlum Steel Corporation, Breckenridge, Pa., a corporation of Pennsylvania No Drawing. Filed May 9, 1963, Ser. No. 2% 6 Claims. (Cl. 117135.1)

This invention relates to the protection of metals and more particularly to protecting ferrous base alloys from oxide scaling or decarburization at elevated temperatures.

In normal processing of ferrous alloys, the metal is heated to elevated temperatures in oxidizing atmospheres. This results in an oxide scale forming on the surface of the alloy. This scale formation means a loss of metal for which reason it is desirable to reduce the scaling. Also, the scale may be rolled into the surface of the product in subsequent operations unless it is completely removed which is another reason that the prevention of scaling is desirable.

It is therefore a principal object of this invention to provide a coating for ferrous base alloys which will minimize formation of scale in oxidizing atmospheres at elevated temperatures.

Yet another object of this invention is the provision of a coating for ferrous base alloys which is easily applied and which will protect the alloy from scaling at elevated temperatures in oxidizing atmospheres and which can be easily removed from the alloy.

Still a further object of this invention is the provision of a method of protecting a ferrous alloy with a coating which will provide a continuous unbroken barrier to the surrounding atmosphere when the alloy is heated in oxidizing atmospheres.

Another object of this invention is to provide a coating which will reduce decarburization of higher carbon ferrous base alloys at elevated temperatures in oxidizing atmospheres.

These and other objects will become apparent from the following description and claims.

It has been found that the amount of oxide scale formed on .a ferrous base alloy when it is heated at elevated temperatures in oxidizing atmospheres can be substantially reduced by coating the surface of the alloy with a mixture of an al ni' and fore the alloy is heated. Upon heati g of the alloy, the coating will protect the surface and substantially reduce the amount of scale which willform. This same type of coating will also reduce the amount of decarburization of higher carbon ferrous base alloys when heated in oxidizing atmospheres.

Alkali metal silicates are required for the coating as opposed to other silicates since these are the only ones that are water soluble and able to be applied as a liquid at ambient temperatures. Of the alkali metal silicates,

3,301,702 Patented Jan. 31, 1967 ice sodium silicate is the most readily available and the most economical, and has been found to provide excellent results. For this reason, sodium silicate is preferred although the others may be used if desired.

It has been found that it is essential that the surface of the alloy must be cleaned thoroughly of grease or oil or the like with an alkaline detergent prior to the application of the coating. If the surface of the alloy is not so-cleaned or is cleaned with an organic solvent such as trichloroethylene, the protective coating will not adhere to the surface even at ambient temperatures as applied and is therefore ineffective to protect at elevated temperatures. When the coating is applied after proper cleaning of the surface, it will substantially reduce the tendency toward oxide scaling of ferrous base alloys even in strongly oxidizing atmospheres in the critical temperature range of from 1800 F. to as high as 2400 E, which temperatures have heretofore produced excessive amounts of scaling on unprotected surfaces of ferrous base alloys. Even stainless steels which are oxidation resistant tend to scale appreciably at these temperatures when their surfaces are uncoated and these coatings are particularly effective in protecting stainless steels in this temperature range.

Table I below shows the composition of four coatin mixtures of sodium silicateand. I. found to be effective in protecting ferrous base alloys. The LL ELwHL-A SDLAY (a trademark of American Cyanamid Company) is added to improve the adherence of the coating.

Coating Sodium Alumina 3 Weight No. Silicate, (-300 mesh), Percent gm. gm. Alumina All coatings contained a 30-120 m1. addition of 10% Aerosol AY wetting agent.

A portion of each of the coatings of Table I was diluted with water and a portion was maintained at full strength undiluted. Samples of types 302 and 430 stainless steel were thoroughly cleaned with Penn-Salt No. 34 (an alkali metal cleaner sold under this trademark by Penn-Salt Chemical Corporation, Philadelphia, Pemisylvania) and were coated at ambient temperatures with each of these coatings and heated to a predetermined temperature in an oxidation balance; the latter is a device which measures the weight gain of the sample due to scaling. Results are listed in Table II. Uncoatcd control samples were also heated to the same temperatures in order to alford a comparison. The efiiciency of the coating can be determined by the amount of weight gain as compared to the weight gain of uncoated control samples.

TABLE II.OXIDATION TESTS ON TYPES 302 AND 430 STEEL WITH ALUMINA-SODIUM SILICATE COATINGS Type 302 Type 430 M thod Tern Wei ht ain (m .lcmJ) during 1 hour exposure Weight gain (mg/cm!) during 1 hour exposure- Coatmg e F g g Coati ng as indicated below Coating as indicated below Not Not Coated No 1 No 2 No 3 No. 4 Coated No 1 No 2 No.3 No 4 .t d t "ce. Diluted 1,800 1. 73 0.07 0.05 0.12 0.08 1.65 0.04 0.08 0.05 0.04 t t aaih g sol t ion. 2, 000 5. 47 1. 19 0.37 0. 49 2. 12 12. 49 0. 31 0. 33 1. 88 5. 08 m iii til. it? 8'33 3%; 3'3? 3%? iii 3 Di coated twice. Full 1,800 1. ,21 s rength coating solution. 2, 000 5. 47 0. 50 0. 45 0.12 0.40 12. 49 1. 08 1. 61 0. 43 0. 56 2, 200 19. 84 2. 59 1. 15 1. 07 9. 78 59. 77 10. 00 16. 10 11. 80 16. 50

The numbers assigned to the coatings are as listed in Table I.

It can readily be seen from an examination of Table II that scaling of both types 302 and 430 stainless steels is considerably reduced as a result of the coating. Within the limits of experimental error, no great difference in protection is observable with different relative amounts of alumina and sodium silicate, although it does appear to be significant that coating No. 4 which has the smallest proportion of alumina, frequently resulted in higher weight gains than the remaining coatings, notably at 2000 F. and above. For this reason, a stronger coating, i.e., 1 one with a greater ratio of alumina to sodium silicate is preferred especially when the alloy is to -be heated to a temperature of 2200 F. or higher.

The equipment available 'for conducting these tests did not permit a quantitative measurement at temperatures greater than 2200 F. However, samples of each of the coatings together with an uncoated sample were heated to 2400 F. and visually inspected. The samples not coated showed appreciably greater scaling than those which were coated, although the coated samples also showed rather extensive scaling. At temperatures above 2400" F. the use of this coating is of little benefit and is not warranted economically. At temperatures below about 1800 F., oxide scaling is not a serious problem.

Five additional coatings were prepared to which were added varying amounts of vermiculite shydrated magnesium-iron-silicate). The various amounts 0 sodium silicate, vermiculite and alumina in each of these coatings are given in Table III below.

Samples of types 302 and 430 stainless steels were thoroughly cleaned with Penn-Salt No. 34, and were coated at ambient temperatures with each of the coatings listed in Table 111 above. The samples were then heated in an oxidation balance as previously described. The results of these tests are given in Table IV below along with those on the uncoated control samples. These results also show that substantial protection is afforded to the coated samples as opposed to uncoated samples wherein the scaling was extensive. However, it was 'found that these coatings which contained vermiculite tended to become quite thick due to the hygroscopic nature of the vermiculite, and water had to be added periodically to maintain the coating sufliciently fluid.

Coatings 5 through 10 containing the vermiculite, had less tendency to crack upon drying than did coatings 1 through 4 which do not contain vermiculite, but these coatings (5 through 10) took longer to dry as a consequence of the increased water content. A comparison of Tables II and IV indicates that the coatings containing vermiculite give somewhat better protection than the coatings containing only alumina, particularly at temperatures of 2000 F. to 2200 F.

The coatings of this invention are useful in protecting not only stainless steel but also other types of steel from scaling at elevated temperatures, whereas uncoated samples of AISI type H-11 die steel heated to 2200' F. and 2300 F. for 1 hour each lost respectively .017 inch and .023 inch from their surfaces as scale. Samples of the same material coated with coating No. 1 (Table I) heated to the same temperatures for the same times with the uncoated samples lost only .005 inch and .009 inch, respectively, from their surfaces as scale.

It has also been observed that upon cooling the coating tended to pop off the type 302 stainless steel, leaving a clean surface. This is believed to be due to the rather high thermal expansion of this austenitic alloy. Thus, there was no need to brush the coating as was necessary with the type 430 stainless steel to completely remove the coating. It was found, however, that simple brushing was adequate to completely remove the coating from the samples of type 430 stainless steel. The pop ofif characteristics of the coating are also excellent if the surface of the alloys to be protected have rusted and the coating is applied directly over this layer of rust. (Of course any lgrease or oil or the like must be thoroughly cleaned from the surface as previously indicated.) The rusted surface does not adversely affect the adherence of the coating or its protection of the surface, but it does appear to have some beneficial effects on the pop off characteristics of the coating.

The coating of this invention is also effective to afford some protection to higher carbon steels, such as high speed tool steels, from decarburization when they are heated to elevated temperatures Three samples of uncoated AISI type M1 high speed steel were heated for 20 minutes, 30 minutes and minutes, respectively, at a temperature of 2075 F. in a gas atmosphere containing about 2% 0 These samples were decarburized to a depth of .016 inch, .017 inch and .028 inch, respectively. Samples of the same grade of material coated with coating No. 1 (Table I) were heated to the same temperature for the same times with the uncoated samples and were decarburized only to a depth of .006 111611, .008 inch and .012 inch, respectively. Hence, it can be seen that this coating is etfective to reduce the depth of decarburization #by about 50% in this material.

Visual examination and qualitative tests on other types of ferrous base alloys have shown that similar protection from scaling and decarburization is afforded although the degree of protection varies depending upon the composition of the alloy.

TABLE IV.OXIDATION TESTS ON TYPES 302 AND 430 STEEL WITH ALUMINA-VERMICULITE-SODIUM SILICATE COATINGS Type 302 Type 430 Weight gain (mg/cm!) during 1 hour exposure-Coating as Weight gain (mgJem. durin 1 hour ex osure-G tin Temp, F. indicated below l1 1dicate 1below p 08 g as Not No. 5 No. 6 No. 7 No. 8 N0. 9 ho. 10 Not No. 5 No. 6 N o. 7 No. 8 No. 9 No. 10 Coated Coated 1. 73 0. 11 0. l4 0. 16 0. 16 (f (1) 1. 0. 08 0. l7 0. 22 0. 40 0. 99 5. 47 0. 16 0. 20 0. 11 0. 24 0. 21 .0. 47 12. 49 0. 15 0. 19 0. 19 0. 26 0. 49 0 19. 84 2. 03 1. 15 0. 0. 56 0. 60 0. 56 59. 77 1. 96 2. 49 0. 0. 69 0. 78 1. 49

All coatings brush coated twice.

The numbers assigned to the coatings are as listed in Table II. tInvalid test due to experimental difficulties.

Although several embodiments of this invention have been shown and described, various adaptations and modifications may be made without departing from the spirit and scope of the appended claims.

We claim:

1. A method of protecting the surface of ferrous base alloys 'from scaling or decarburization in oxidizing atmospheres at temperatures from 1800 F. to 2400 F., comprising the steps of cleaning the surface of the alloy with an inorganic detergent cleaner, and thereafter placing on the surface of the alloy a coating consisting essentially of an aqueous solution of an alkali metal silicate and from about 14% to about 53% aluminum oxide.

2. The method of claim 1 wherein the surface of the alloy is provided with a layer of rust prior to coating.

3. The method of claim 1 wherein the alkali metal silicate is sodium silicate.

- 4. A method of protecting the surface of ferrous base alloys from scaling or decarburization in oxidizing atmospheres at temperatures from1800 F. to 2400" F., comprising the steps of cleaning the surface of the alloy References Cited by the Examiner UNITED STATES PATENTS 1,750,305 3/1930 Gross 1l7135.1 X 2,364,436 12/1944 Frisch 117135.l 2,384,542 9/1945 Fruth et a1. 2,485,061 10/1949 Nebe 10'684 X 2,711,974 6/1955 Happe 106-84 X 2,832,705 4/1958 Seidl 117--169 X RALPH S. KENDALL, Primary Examiner. 

1. A METHOD OF PROTECTING THE SURFACE OF FERROUS BASE ALLOYS FROM SEALING OR DECARBURIZATION IN OXIDIZING ATMOSPHERES AT TEMPERATURES FROM 1800*F. TO 2400*F., COMPRISING THE STEPS OF CLEANING THE SURFACE OF THE ALLOY WITH AN INORGANIC DETERGENT CLEANER, AND THEREAFTER PLACING ON THE SURFACE OF THE ALLOY A COATING CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF AN ALKALI METAL SILICATE AND FROM ABOUT 14% TO ABOUT 53* ALUMINUM OXIDE. 